One of the most important parameters in order to obtain maximum solar radiation for solar power system is the optimum tilt angle of photovoltaic (PV) panels. In this article, the optimum tilt angles of photovoltaic panels are determined for regions of different climatic zones. The annual maximum energy output of PV panels is calculated in MATLAB considering irradiance data only. Annual, seasonal and monthly optimum tilt angles of PV arrays of approximately same latitude with different climatic zones are calculated for fixed tilt angles on the basis of maximum energy output. The incident global solar radiation on the horizontal surface is separated to its direct and diffuse components with a solar angle based mathematical model. Depending on region’s latitudes, direct and diffuse radiation, and tilt angle of PV panels, the incident global radiation on inclined surface is calculated for USA conditions. This study is conducted for seven different regions with very close latitudes but different climate zones. It is clear from the result that PV panels with fixed tilt angles located in approximately-same latitudes may result in different optimum tilt angle due to the dissimilar climatic impacts.